1. Field of the Invention
The present invention relates to a telephone hold circuit, and more particularly to a telephone hold circuit manufactured in a dialer integrated circuit.
2. Description of Related Art
As the number of telephones in each household increases, so does the need to provide each telephone with its own telephone hold circuit. There are some known general arrangements that can be used to provide a telephone with hold function.
According to a first such arrangement, a separate telephone hold circuit is connected to the telephone's telephone dialer integrated circuit (IC). Referring to FIG. 1, this kind of telephone hold circuit includes a low-power silicon controlled rectifier (SCR) 10, a light-emitting diode (LED) 11, a hold button 12 and a plurality of resistors 13 to 15. Under normal condition, the SCR 10 is electrically open-circuit, so the hold circuit is not active and the light-emitting diode 11 is dark since no current flows therethrough. As soon as the hold button is depressed, the ringing voltage is conducted to the gate of the SCR 10 to activate the SCR 10. Then, the SCR 10 conducts current and the light-emitted diode 11 lights. The ringing voltage is also sent to the tip wire through the conductive SCR 10. Therefore, a call can be placed on hold even if the handset is on-hook after depressing the hold button.
However, this approach increases the number of components required and hence the complexity and cost of the telephone. Further, it is difficult to set up the telephone hold circuit since the values of the resistors have to be carefully selected so as to prevent malfunction due to interference of noises on the telephone line.
According to a second arrangement, the telephone hold circuit is incorporated into the telephone dialer IC. This approach increases the size of the telephone dialer IC because two additional pins (a tri-state input device and a hold function pin) are required. Moreover, the hold function input signal employed in the second arrangement is not derived from the button keypad of the telephone, and thus does not undergo a de-bouncing process to obviate any erroneous signal. Therefore, erroneous signals may cause the telephone to go on hold or off hold when not intended.
The hold circuit of a typical dialer IC is illustrated in FIG. 2a. An external circuit 20 having a transistor 30 and a switch 40 is coupled to the hold circuit. The switch 40 is mounted on the telephone panel (not shown) to serve as a hold button. The input port labeled HK is used to input the off-hook state or on-hook state of the telephone handset. The state of HK is logical "0" while the handset is in the off-hook state, that is, the handset is picked up. The state of HK is logical "1" when the handset is in the on-hook state, that is, the handset is put down.
The output signal HR of the NOR gate 52 will be a pulse signal when the handset is in the off-hook state because the signal HR is obtained by the logical NOR operation of the HK signal and an inverted and delayed HK signal obtained by passing the HK signal through a delay circuit 50 and an inverter 51. As shown in FIG. 2b, when the handset is being picked up at time t1, the state of HK is changed from logical "1" to logical "0". Because of the logical NOR operation of the HK signal and an inverted and delayed HK signal obtained by passing the HK signal through a delay circuit 50 and an inverter 51, the signal HR is a pulse during the period from time t1 to time t2.
The output signal HR outputted from NOR gate 52 is used to convert the hold status. Therefore, the hold output HOLDOUT is a logical "0" when a pulse signal inputs to the flip-flop 53. Whenever the hold button is depressed, the hold status changes once. In addition, when the extension handset is picked up, it converts the hold status by a low level pulse generated by the emitter of the transistor 30 due to a variation of the voltage pulse Ep from the telephone line.
The above-described arrangement, however, has the following drawbacks. First, a variation of the voltage Ep from the telephone line is used to release the hold status, which may also result in a mistaken change of status of the telephone from non-hold to hold status. Secondly, since the characteristics of the noise produced by the telephone line and the bouncing noise produced by the push button mechanism while the push buttons are depressed are different, simply adding a capacitor on the port HOLDIN which is employed to input the telephone hold signal cannot completely eliminate the above two kinds of noise. Therefore, the telephone hold function will not be stable.